Leupeptin, a protease inhibitor, will be used to quantify&characterize a variety of proteases produced in cell cultures. Initially it will be used to detect a protein produced in the ng/ml range by human T lymphocytes in culture. This low concentration requires leupeptin to be labelled at a very high specific activity. Subsequently 3H leupeptin will be used to analyze libraries of variant enzymes with different functional properties created through protein engineering techniques.